Clouds can present risks to aircraft when traveling through them. When in a cloud, ice can form on control surfaces and/or lift surfaces. When aircraft engines ingest excessive moisture, the chemistry of combustion can change. Large ice particles can be abrasive to exposed surfaces of aircraft engines. And very large ice particles can even damage aircraft engines. Not every cloud, however, presents these hazards to an aircraft. Different clouds and different atmospheric conditions may be accompanied by various water droplet size distributions, different ice/liquid ratios, etc., some of which may be entirely safe to an aircraft. Such water droplet size distributions and ice/liquid ratios may be measured as cloud metrics using various instruments.
Some aircraft are equipped with these instruments to detect ice accretion on an exterior surface of the aircraft. Magnetostrictive resonators have been used for such purposes. A resonant frequency of the magnetostrictive resonator changes in response to ice accumulation on a resonator. The ice accumulation changes the mass of the resonator, which in turn changes the frequency of resonance. Liquid water, in contrast to accreted ice, does not fixedly attached to an exterior surface of an aircraft. Direct methods of detecting liquid water on an aircraft surface have been more difficult to perform.